Since you're not sure that he has it, you should visit a genetic specialist for a chromosome screening. If you'd like to see if it is possible to have another child with DMD, then the mother could get a chromosome screening as well. This would only be useful for women. Since DMD only effects the X chromosome, a male can't unknowingly carry DMD because males have an X and Y - both are actively used - where women have X and X, but only one is actively used, so if DMD is carried in only one of the X's, a women still has one copy of the correct genes.
Does Your Son have Duchenne Muscular Dystrophy?
Since you're not sure that he has it, you should visit a genetic specialist for a chromosome screening. If you'd like to see if it is possible to have another child with DMD, then the mother could get a chromosome screening as well. This would only be useful for women. Since DMD only effects the X chromosome, a male can't unknowingly carry DMD because males have an X and Y - both are actively used - where women have X and X, but only one is actively used, so if DMD is carried in only one of the X's, a women still has one copy of the correct genes.
Here is a support group for DMD
http://www.mdjunction.com/duchenne-muscular-dystrophy
Here is a video that a sister made to inspire those with family members who have DMD, like her.